Generally, efficiencies in direct-fired industrial processes, for example, the making of glass are limited by the mechanics of heat transfer. Heat transfer is relatively efficient for thermal loads in the vicinity of the flame. However, in practical applications the thermal load is not located concentrically around the flame as would be desirable for efficient heat transfer. Instead, most thermal loads accept heat flux through planar or moderately curved surfaces such as the horizontal surface of a glass/metal melt in a glass/melt furnace, the vertical wall of a heat exchanger in a boiler or the curved surface of a charge of granular material in a rotary-kiln furnace.
It is known that the addition of a reaction-rate enhancing gas improves the heat transfer of a direct-fired system. The term "reaction-rate enhancing gas" as used herein includes any gas that will influence the desired reaction in a positive manner, for example, oxygen, oxygen-enriched air, mixtures of oxygen and other gases such as acetylene, and the like. This is because the flame temperature and thereby the transfer of heat from the flame to the thermal load will be increased. Additionally, the reduction of nitrogen flowing through the combustion system diminishes exhaust loses.
Although the addition of a reaction-rate enhancing gas increases heat transfer efficiency, there have been efforts to improve heat transfer by altering the relative positioning of the burner and the pipe or lance used to transmit the rate-enhancing gas to the system. Conventionally, an oxygen jet flow is introduced between the flame and the thermal load through an axisymmetirc lance with the ejection port of the lance typically positioned adjacent the burner. As used herein, the term "axisymmetric" means that a cross-section of the gas jet or flame at the point of exit from the lance or burner taken perpendicular to the longitudinal axis of the flow of the jet or flame is essentially circular. "Nonaxisymmetric" means that the cross-section is essentially non-circular.
In such systems, the introduction of oxygen using an axisymmetirc lance results in the formation of a narrow, pencil-shaped, high-temperature zone along the length of the flame. The heat flux to the thermal load is greater from the oxygen-intensified zone than from the rest of the flame. As a result, the thermal load is exposed to high localized heat fluxes which result in unevenly heated areas in the thermal load, conventionally referred to as "hot spots". Uneven heating limits the efficiency and capacity of the furnace and adversely affects product quality.
Another method of improving heat transfer is disclosed in Bienus et al U.S. Pat. No. 4,444,586. An axisymmetric oxygen/fuel lance is directed perpendicular to the melt surface in a reverberatory furnace for melting copper so that the flame is in head-on contact with a portion of the copper. The flame is produced by an axisymmetric oil atomizer with a (concentric) coannular oxygen supply. This arrangement produces enhanced heat transfer only very local to the area of flame impingement on the copper.
The average static pressure in a jet of a reaction-rate enhancing gas is lower than the ambient pressure; how much lower being dependent on the density and velocity of the jet flow. This observation can be derived from a quantitative analysis of the governing momentum equations.
Virtually all flames produced in industrial furnaces are essentially isobaric (low Mach number). This can be ascertained by focusing on the flow across the flame on a length scale of the order of the flame thickness. In such an analysis, flame curvature can be neglected, even for turbulent flames. A standard order-of-magnitude analysis leads to the conclusion that the viscosity effects on the pressure drop across the flame can be neglected as well.
Applicant has discovered that the reduced static-pressure field induced by a dense (cold) jet can efficiently displace and deform an isobaric, less dense hot flame. By appropriately positioning the jet between the thermal load and the flame, and by employing a nonaxisymmetric jet, the flame is drawn toward the low static-pressure field induced by the colder jet. The flame is also deformed toward the nonaxisymmetric shape of the cold jet. This results in more favorable flame placement and geometry control which increases the efficiency of the flame to transfer heat to the entire thermal load without the formation of hot spots.
The term burner as generally known and used herein describes a device which emits a fuel or a fuel in combination with an oxidant. The lance emits a rate-enhancing gas in the desired proximity to the flame generated with said burner.
The burner disclosed in accordance with the present invention includes not only a fuel outlet to emit a fuel alone or a fuel in combination with an oxidant, but also a gas lance through which a reaction rate-enhancing gas is emitted.
In either case, the desired objects of the invention can be obtained by use of the above-described novel lance/burner combination or the novel burner.
In addition, a nonaxisymmetric shaped fuel outlet or burner outlet may be employed, preferably having a shape similar to the lance. As a result, the fuel as well as the gas jet momentum and relative flame direction allow greater control over the flame position, shape and temperature which leads to even more favorable heat-transfer characteristics over conventional lancing arrangements and techniques.
It is therefore an object of the invention to provide a method of more uniformly enhancing the heating of a thermal load by employing an improved lance/burner apparatus.
It is a further object of the invention to provide a lance alone or in combination with a fuel burner or incorporated as part of a burner capable of ejecting a nonaxisymmetric profile of gas to interact with an axisymmetric or nonaxisymmetric flame to thereby deform and displace the flame and increase the efficiency of heat transfer thereof over the entire thermal load.
It is a further object of the invention to provide an lance/burner combination which produces a flame which can uniformly heat a thermal load without the formation of hot spots.
It is another object of the invention to provide a flame with the desired characteristics through the use of a burner containing a lance.
It is an additional object of the invention to provide a furnace including a lance/burner combination or a burner having a lance incorporated as part of the burner capable of producing a flame which uniformly covers the thermal load and is more efficient in heat transfer than known systems.